Editor’s Note: Today’s caption is the answer to Earth Observatory’s March Puzzler. The Namib Desert
in southwestern Africa is one of the most inhospitable places on the
planet. It rarely rains, unless you count the fog that often blankets
the coastal area. There are no lakes or ponds. There’s almost no surface
water at all, save for ephemeral streams that emerge when scattered
storms bring rain in the winter. Even those streams flow for just a few
days or weeks each year, and they disappear into the ground or evaporate
into the air long before they reach the sea.Given the scarcity of water, it’s not surprising that Namibia has the
second lowest population density in the world, trailing only Mongolia.
And the Namib Desert is one of the most sparsely-populated parts of
Namibia. But despite the harsh environment, there is still something
that draws people deep into the desert: uranium. Namibia is among the
top five uranium-exporting countries in the world.The Erongo region is home to Rössing mine,
the oldest and third-largest producer of uranium in the world. Situated
about 70 kilometers (40 miles) northeast of the coastal city of Swakopmund, the mine is located near the ephemeral Khan River. The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite acquired this image of the mine and surrounding landscape on March 8, 2013.Roessing’s 400-meter (1,300-foot) deep pit and a large processing
facility north of it are the dominant features in the bottom image. The
pit taps into layers of metasedimentary rock that contain intrusions of a type of coarse-grained igneous rock called pegmatite. Pegmatite often contains rare minerals and forms when magma slowly cools and hardens beneath the surface.
The mine sustains the small satellite town (population 7,600) of Arandis,
which is visible near the top of the image. Roads and rail connect
Arandis to Swakopmond, and there is a small airport south of Arandis.
The dried channel of the Khan River is visible near the bottom of the
image. While the surrounding landscape is largely devoid of vegetation,
groundwater beneath the channel sustains some shrubs, trees, and
grasses. The patches of green within and around the river channel in the
top image are vegetation. Some Namibians and environmental groups have raised concerns
that water samples from the Khan River channel showed elevated uranium
levels. Uranium carried in wastewater from the mine may be reaching
groundwater and increasing uranium levels near the river. However, a
study led by Michael Schubert,
a scientist from the Helmholtz Center for Environmental Research, found
that uranium also occurs naturally in the channel sediments. His team
found no evidence that water contaminated by the mine had seeped into
the groundwater.

On March 24, 2013, a dust storm arose
on the Arabian Peninsula, blowing through parts of Saudi Arabia,
Kuwait, and Iraq. Dust also stretched across the Persian Gulf toward
Iran. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured this natural-color image the same day.This dust storm was part of a larger pattern of activity, occurring one day after a dust storm in Iraq, and two days after a dust storm in Egypt.Lee Grenci, a
meteorologist at Penn State University, explained that the large-scale
atmospheric disturbance moved from west to east across northeastern
Africa and the Middle East from March 22–24, 2013. The disturbance
occurred at roughly 18,000 feet (5,000 meters) in altitude. Meanwhile,
increasing amounts of late-March sunshine heated the land surface and
led to an overturning in the atmosphere near the ground, such that upper
level winds mixed with weaker winds near the ground. Grenci likened the
events to mixing oil and vinegar to make salad dressing. “Like the
salad dressing, the wind profile in the lowest several thousand feet
then became more homogeneous, with winds near the Earth’s surface
becoming stronger and gustier, which caused the blowing sand and dust.”Besides the atmospheric disturbance, the Arabian peninsula had
another ingredient necessary for dust storms: sand. Uninhabited sandy
deserts comprise most of Saudi Arabia’s land surface, and less than 2
percent of the land is arable.